The present invention relates to a novel apparatus and method for delignifying and bleaching lignocellulosic pulp with a gaseous bleaching agent such as ozone. More particularly, the present invention includes a first stage where pulp and ozone are subjected to high shear mixing and a bleaching reaction occurs, and a second stage where the mixed pulp is held in a retention bed for further reaction and stripping of the ozone from the carrier gas.
To avoid the use of chlorine as a bleaching agent for pulp or other lignocellulosic materials, the use of ozone in the bleaching of chemical pulp has previously been attempted. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost have previously limited the development of satisfactory ozone bleaching processes for lignocellulosic materials in general and especially for southern softwoods.
Ozone will readily react with lignin to effectively reduce the amount of lignin in the pulp, but it will also, under many conditions, aggressively attack the carbohydrate which comprises the cellulosic fibers of the wood to substantially reduce the strength of the resultant pulp. Ozone, likewise, is extremely sensitive to process conditions such as pH with respect to its oxidative and chemical stability. Changes in these process conditions can significantly alter the reactivity of ozone with respect to the lignocellulosic materials.
Since the delignifying capabilities of ozone were first recognized around the turn of the century, there has been substantial and continuous work by numerous persons in the field to develop a commercially suitable method using ozone in the bleaching of lignocellulosic materials. Furthermore, numerous articles and patents have been published in this area and there have been reports of attempts at conducting ozone bleaching on a non-commercial pilot scale basis. For example, U.S. Pat. No. 2,466,633 to Brabender et al., describes a bleaching process wherein ozone is passed through a pulp having a moisture content (adjusted to an oven dry consistency) of between 25 and 55 per cent and a pH adjusted to the range of 4 to 7.
U.S. Pat. No. 3,814,664 to Carlsmith discloses a gaseous reaction apparatus including a peripheral gas receiving chamber which is said to be useful for ozone bleaching of pulp. Pulp to be bleached is fed through a tapered, compacting plug conveyor to create a gas seal. The plug is then broken up by a screw disintegrator at which point ozone from a gas tight vessel is mixed with the pulp. The pulp is fluffed and fibers are entrained in the gas which is then directed into a pulp bed in the gas tight vessel for reaction of the pulp and ozone. The carrier gas is removed through an annular discharge chamber and the pulp is retained for at least twenty minutes to allow completion of the bleaching reaction. Dilution liquid is supplied into the bottom of the tank with the pulp and the pulp is discharged from the tank when the reaction is complete.
Two other patents to Carlsmith disclose vessels for containing gaseous reaction beds. In U.S. Pat. No. 3,785,577, pulp is supplied to the vessel first through a compacting screw and then a feed screw, into a mechanism for breaking up the compacted pulp and spreading it across the cross-section of the vessel. The reaction gas is supplied by separate conduit into the vessel. The reacted pulp is removed from the bottom of the vessel by a screw mechanism which forces the pulp through a flap means to recompact the reacted pulp. U.S. Pat. No. 3,964,962 discloses a modification to the '664 and '557 patents discussed above. A gas release zone is provided on the vessel for receiving discharged gas. A system is then provided to direct at least a portion of the discharged gas back into the top of the vessel to supplement the new reaction gas supplied.
Other ozone bleach sequences are described by S. Rothenberg, D. Robinson & D. Johnsonbaugh, "Bleaching of Oxygen Pulps with Ozone", Tappi, 182-185 (1975)--Z, ZEZ, ZP and ZP.sub.a (P.sub.a -peroxyacetic acid); and N. Soteland, "Bleaching of Chemical Pulps with Oxygen and Ozone", Pulp and Paper Magazine of Canada, T153-58 (1974)--OZEP, OP and ZP. Further, U.S. Pat. No. 4,196,043 to Singh discloses a multi-stage bleaching process utilizing ozone and peroxide which also attempts to eliminate the use of chlorine compounds, and includes recycling of effluents.
Various bleaching apparatus utilizing a central shaft with arm members attached thereto are generally known (see, e.g., U.S. Pat. Nos. 1,591,070 to Wolf, 1,642,978 and 1,643,566, each to Thorne, 2,431,478 to Hill, and 4,298,426 to Torregrossa et al.). Also, U.S. Pat. Nos. 3,630,828 to Liebergott et al. and 3,725,193 to de Montigny et al. each disclose a bleaching apparatus for use with pulp having a consistency of above 15 percent, which apparatus includes a rotating shaft having radially spaced breaker arms for comminuting the pulp. Richter U.S. Pat. No. 4,093,506 discloses a method and apparatus for the continuous distribution and mixing of high consistency pulp with a treatment fluid such as chlorine or chlorine dioxide. The apparatus consists of a concentric housing having a cylindrical portion, a generally converging open conical portion extending outwardly from one end of the cylindrical portion, and a closed wall extending inwardly from the other end of the cylindrical portion. A rotor shaft mounted within the housing includes a hub to which a plurality of arms are attached. These arms are each connected to a transport blade or wing. Rotation of the shaft allows the treatment fluid to be distributed in and mixed with the pulp "as evenly as possible."
Fritzvoid U.S. Pat. No. 4,278,496 discloses a vertical ozonizer for treating high consistency (i.e., 35-50%) pulp. Both oxygen/ozone gas and the pulp (at a pH of about 5) are conveyed into the top of the reactor to be distributed across the entire cross-section, such that the gas comes in intimate contact with the pulp particles. The pulp and gas mixture is distributed in layers on supporting means in a series of subjacent chambers. The supporting means includes apertures or slits having a shape such that the pulp forms mass bridges thereacross, while the gas passes throughout the entire reactor in contact with the pulp.
Displacement of pulp through the reactor takes place by the repeated but controlled breaking of the supporting means by the rotation of the breaking means which are attached to and rotated by a central shaft. This allows the pulp to pass through the apertures and into the subjacent chambers. Fritzvoid et al. U.S. Pat. No. 4,123,317 more specifically discloses the reactor described in the aforementioned Fritzvoid '496 patent. This reactor also is used for treating pulp with an oxygen/ozone gas mixture.
U.S. Pat. Nos. 4,468,286 and 4,426,256 each to Johnson disclose a method and apparatus for continuous treatment of paper pulp with ozone. The pulp and ozone are passed along different paths either together or separately.
U.S. Pat. No. 4,363,697 illustrates certain screw flight conveyors which are modified by including paddles, cut and folded screw flights or combinations thereof for use in the bleaching of low consistency pulp with oxygen.
French Pat. 1,441,787 and European patent Application 276,608 each disclose methods for bleaching pulp with ozone. European Pat. Application No. 308,314 discloses a reactor for bleaching pulp with ozone utilizing a closed flight screw conveyor, wherein the ozone gas is pumped through a central shaft for distribution throughout the reactor. The pulp has a consistency of 20-50% and the ozone concentration of the treating gas is between 4 and 10% so that 2 to 8% application of ozone on O.D. fiber is achieved.
In general, the prior art has not achieved a successful reactor or method for ozone bleaching of pulp which provides a substantially uniformly bleached pulp. In slowly moving retention-type beds, such as in the Carlsmith devices discussed above, some of the pulp is isolated from the gaseous bleaching mixture relative to other pulp due to differences in bed height and bulk density at various positions within the bed. This causes non-uniform passage of the bleaching mixture gas through the fiber bed, which in turn results in non-uniform ozone-pulp contact and non-uniform bleaching. Also, mixing at low to medium pulp consistencies is undesirable because of the substantially greater amounts of ozone required to achieve the same level of bleaching due to the ozone being diffused through water.